(1) Field of the Invention
The present invention relates to a ringing signal generation circuit having a function to protect against an overvoltage in the generated ringing signal.
In telephone exchange systems, ringing signal number circuits are provided for generating a ringing signal to inform a called party that a call is incoming, by a ringing tone. The ringing signal is sent to a telephone set at the called party through a subscriber circuit and a telephone line.
Usually, the ringing signal generation circuit is constructed using a DC-DC converter such as a ringing choke converter, and an invertor to generate a high voltage alternating signal from a low voltage direct current voltage source.
(2) Description of the Related Art
FIG. 1 shows a conventional construction of a ringing signal generation circuit. In FIG. 1, reference numerals 1, 7, 8, 11, 12, 30, 31, 34, 35, 36, 37, 39, 41, 42, and 44 each denote a resistor, 2 denotes a primary winding, 3 denotes a direct current (DC) voltage source, 4 denotes a switching transistor, 6 denotes a base-current control transistor, 5, 9, 17, 20, 24, 25, and 38 each denote a diode, 13, 18, 21, 23, 27, and 43 each denote a capacitor, 14 denotes a base-driving winding, 15 denotes a transformer, 16 and 26 each denote an additional winding, 19 and 22 each denote a secondary winding, 28 and 34 each denote an output control transistor, 29 and 33 each denote a phototransistor, 40 and 46 each denote a transistor, 47 denotes a relay, 48 denotes an oscillator, 49 and 50 each denote a photodiode, 60 denotes a power source of a control circuit, and 70 denotes a relay switch.
The primary winding 2, the direct current (DC) voltage source 3, the resistors 1, 7, 8, 11, and 12, the switching transistor 4, the base-current control transistor 6, the diodes 5 and 9, the Zener diode 10, the capacitor 13, and the base-driving winding 14, constitute a primary side of a ringing choke converter.
In the secondary side of the transformer 15, two secondary windings 19 and 22 and two additional windings 6 and 26 are provided. To each of the windings 16, 19, 22, and 26 in the secondary side, a rectifying and smoothing circuit comprised of a diode (17, 20, 24, or 25) and a capacitor (18, 21, 23, or 27) is connected.
In the secondary side, the secondary winding 19 and the rectifying and smoothing circuit (20, 21) which is connected to the secondary winding 19, are provided to supply the output therefrom for generating a positive-voltage component of the ringing signal, and the secondary winding 22 and the rectifying and smoothing circuit (23, 24) which is connected to the secondary winding 22, are provided to supply the output therefrom for generating a negative voltage component of the ringing signal. To generate the AC ringing signal, an invertor circuit is provided in the output side of the rectifying and smoothing circuits (20, 21) and (24, 23). The invertor circuit is comprised of the output control transistors 28 and 32. The output control transistors 28 and 32 are respectively controlled by the phototransistors 29 and 33, and the phototransistors 29 and 33 respectively receive a power supply from the additional windings 16 and 26 through the rectifying and smoothing circuits (17, 18) and (25, 24).
The phototransistors 29 and 33 are respectively driven by light emitted from the photodiodes 50 and 49 in a control circuit, and the phototransistors 50 and 49 alternatively emits light responding to positive and negative voltage components of an analog AC output current having a frequency of 20 Hz from the oscillator 48.
The ringing choke converter is known as a self-oscillating DC-DC converter which can be constructed by a small number of circuit components and at a low cost. However, the operation of the ringing choke converter depends much on the characteristics of the circuit components such as the switching transistor, the Zener diode, the capacitor and the like.
For example, the output voltage of the ringing signal generation circuit may exceed much over a tolerable range of the ringing signal (overvoltage) due to a deterioration caused by aging of one or more circuit components. The overvoltage in the ringing signal may cause a burning of a circuit component in the path of the ringing signal in the subscriber circuit.
The construction comprised of the resistors 39, 37, 41, 42, and 44, the capacitor 43, the Zener diode 45, the transistors 40 and 46, the relay circuit, and the relay switch 70, is provided for generating an alarm signal when an overvoltage is detected in the output of the ringing signal. The above-mentioned high-voltage-side output terminal of the rectifying and smoothing circuit (20, 21) is also connected to an end of a series connection of the resistor 39, the three diodes 38, and the resistor 37 (an end of the resistor 39) to monitor an overvoltage in the output of the rectifying and smoothing circuit (20, 21). The connecting point between the diodes 38 and the resistor 37 is connected to the base terminal of the transistor 40. The collector of the transistor 40 is connected to an end of the resistor 41, the other end of the resistor 41 is connected to an end of the capacitor 43, and the other end of the capacitor 43 is connected to the emitter of the transistor 40. The high-voltage-side terminal of the capacitor 43 is connected to the base terminal of the transistor 46 through the Zener diode 45. The transistor 46 is connected in series in a circuit for applying power to the relay circuit 47 which is provided for controlling an overvoltage alarm. When the voltage of the high-voltage-side output terminal of the rectifying and smoothing circuit (20, 21) is normal, the voltage of the connecting point between the diodes 38 and the resistor 37, i.e., the voltage of the base of the transistor 40 is low. Therefore, the transistor 40 is OFF, the voltage of the high-voltage-side terminal of the capacitor 43 is high, and the current flows through the Zener diode 45 to the base of the transistor 46. Receiving the current to the base, the transistor 46 is normally ON, a current from the voltage source 60 flows in the relay circuit 47, and correspondingly the relay switch 70 is normally OFF. When the voltage of the high-voltage-side output terminal of the rectifying and smoothing circuit (20, 21) exceeds a predetermined voltage level, e.g., 170 V, the voltage of the connecting point between the diodes 38 and the resistor 37, i.e., the voltage of the base of the transistor 40 becomes high. Therefore, the transistor 40 becomes ON, the voltage of the high-voltage-side terminal of the capacitor 43 becomes low, and the current does not flow through the Zener diode 45 to the base of the transistor 46. Therefore, the transistor 46 becomes OFF, the current does not flows in the relay circuit 47, and correspondingly the relay switch 70 becomes ON, i.e., an alarm signal is output.
However, in the above construction, an overvoltage is detected and is informed to the operator as an alarm signal, but the overvoltage itself is not controlled, and therefore, there remains a risk that the circuit component in the path of the ringing signal in the subscriber circuit, is damaged.